Social Networking

Microblogging

Twitter, Tumblr
A Blog is a contraction of 'weB log'.
Usually a blog has text and may have images.
It is about presenting a point of view or information in which you have some interest.
A microblog has more limited amount of text.

Publishing

Wordpress - a full featured Blogging application with a reasonable learning curve.

Blogger - A free Google product that is easy to use.

These are typical Blog services that allow much more text, multiple pages and other features.
You may provide options for comments from readers.

Collaboration

Wikipedia
Wikipedia is an example of a website where content is generated in a collaborative way.
This is an online encyclopaedia that is created by the efforts of multiple people who are able to edit each other's work.

Photosharing

Flikr - shared images that have a range of re-use options depending on copyright. It is also used for showcasing photography skills.
Instagram - shared images and video, almost a visuals only version of Facebook
Pinterest - the idea is an electronic pinboard of special interest images that are grouped by users
Flipboard - is a site that "aggregates" (or gathers) content from multiple sites and presents it in interest based magazine format
These sites are primarily about sharing images but in vastly differing ways.
The sites are also structured in different ways that are focused on the user base.

Videosharing

Video sharing can be about entertainment, education, training, self-promotion, special interest sharing or skill showcasing
Videos can be of varying quality from home-made using mobile phones to 'short films' using high end video cameras.

YouTube - Google owned, this is a repository of enormous diversity in material and useage. It also has substantial variation in video quality.

Vimeo - requires a minimum standard of video quality. Collects more the professional end of videographic material.

Virtual Worlds

These are essentially an alternate reality that is intended for entertainment, experiment, gaming and business.

Second Life Click here - probably closer to the serious end of the possibilities, second life can be used for conferencing, education and training in a safe environment for people interactions. Requires a relatively powerful computer to make the 'worlds' appear as smooth video.

Farmville Click here - much closer to the gaming end, this is a simulation of a farm experience in a more 'cartoon' environment.

News Feed / Social Bookmarking

Digg

Delicious

Group Buying

Groupon

Review networks

These are sites that attempt to collate many reviews to give purchasers of goods or services an insight into opinion about goods or services discussed.
Many sites encourage reviews to drive interest for return visits. Reviewers are given ratings depending on the quality and depth of reviews which again encourages return.

TripAdvisor - many reviews of locations, accommodation and sites of interest around the world. Basically focused on travel reviews.